The present invention relates to a laser beam focusing head, in particular, a focusing head essentially presenting a lens for focusing a laser beam, via an output, on to a given area in which, in practical use, a workpiece is located for subjection to laser beam radiation.
A serious drawback on known focusing heads of the aforementioned type is the rapid deterioration of the lens surface caused by molten material splash, vapour or fumes from the workpiece subjected to laser beam radiation. Such a drawback is particularly encountered when welding so-called "effervescent" materials, or materials reacting violently with the atmosphere or cover gas employed, e.g. aluminium, zinc, titanium or faced or composite materials with adhesives inbetween, etc. When heated violently by an incident laser beam, such materials react by giving rise to a sort of explosion in which incandescent material is splashed on to the surface of the lens. The same applies even when welding workpieces containing only a small percentage of effervescent material as, for example, in the case of coated sheet metal. When welding overlapping sheets of coated metal, the material inbetween has been found to vaporize rapidly and even to splash off part of the molten metal on the sheet itself.
The main problem resulting from such deterioration of the focusing lens is that the latter has to be changed frequently, at times, after only a few minutes' operation. As a result, focusing heads of the aforementioned type are not cheap to use; firstly, because the lens is relatively expensive as compared with the head; secondly, because of the frequent holdups required for changing the lens; and, thirdly, because even only partial damage to the lens impairs the reliability of the process as a whole.
One attempt already made to overcome the aforementioned drawback is to supply a jet of inert gas for protecting the area affected by the laser beam. As, in the case of welding, however, the said gas jet must be light enough to prevent affecting the weld pool, the force of the said jet is often insufficient for preventing incandescent material from being splashed onto the surface of the lens.